1. Field of the Invention
The present invention relates generally to methods and compositions for altering the permeability of subterranean formations, and more particularly, to methods and compositions for forming in situ gels in water producing and/or waterflooded subterranean formations.
2. Description of the Prior Art
Undesirable water recovered from oil wells can result from the infiltration of naturally occurring subterranean water or, in the case of waterflooding operations, from the injected drive water. In both cases, water flows through zones of high permeability to the producing well bores whereby oil in less permeable zones is bypassed.
In waterflooding operations, the more permeable zones of subterranean formations tend to take most of the injected flood water. While this is acceptable initially when the oil contained in the high permeability zones is being swept therefrom, it subsequently becomes undesirable as the oil in such zones becomes depleted. From that point on, the water-flood provides little benefit in enhancing oil production.
Methods and compositions for accomplishing near well and in depth plugging of high permeability zones by forming gels therein have heretofore been developed and used. The formation of the gels causes high permeability zones to be plugged or at least lowered in permeability whereby subsequently injected flood water is caused to enter previously bypassed zones. This in turn causes the flood water to mobilize increased amounts of oil which are recovered from the subterranean formation.
A variety of methods and polymer compositions which gel in situ to reduce the permeability of high permeability zones in subterranean formations have been utilized successfully. U.S. Pat. No. 3,762,476 issued on Oct. 2, 1973 discloses a method of reducing recovered water from a subterranean formation wherein aqueous polymer solutions interspaced with aqueous solutions of crosslinking metallic ions are injected into the formation. A variety of polymers can be utilized in conjunction with various multivalent metal cations complexed with sequestering agents selected from acetate, nitrilotriacetate, tartrate, citrate and phosphate. The solution containing the crosslinking complex is injected after the first polymer solution injection, followed by injection of further polymer solution, which sequence can be repeated.
U.S. Pat. No. 4,569,393 issued on Feb. 11, 1986 discloses a water permeability correction process to improve the sweep efficiency of waterflooding which involves the sequential injection of a solution containing a sequestered polyvalent metal cation, such as aluminum citrate, and a polymer solution containing a gelable polymer, such as polyacrylamide, followed by the injection of carbon dioxide to decrease the pH of the polymer which activates the delayed in situ gelation thereof. The field applications of this method and the method described in U.S. Pat. No. 3,762,476 are restricted to fields having an available fresh water supply because the crosslinking metal ion complexes and/or the gelation activator are incompatible with hard brines.
U.S. Pat. No. 4,552,217 issued on Nov. 12, 1985 discloses a water permeability correction process to improve the sweep efficiency of waterflooding which involves the sequential injection of an aqueous solution containing polyvalent metal cations sequestered with biocidal anionic sequesterants, and an aqueous solution containing a gelable polymer such as polyacrylamide.
U.S. Pat. No. 4,683,949 issued on Aug. 4, 1987 describes a conformance improvement method using a gel comprised of a polyacrylamide polymer and a chromium (III) acetate gelling agent capable of crosslinking the polymer in an aqueous solvent. While the above-described methods and compositions as well as other methods and compositions have been utilized successfully for improving the sweep efficiency of waterflooding and/or reducing the production of naturally occurring formation waters, such methods and compositions have generally been unsuccessful in applications requiring the formation of gels deep in high permeability zones having high temperatures, i.e., temperatures above about 160.degree. F. At such temperatures, the heretofore used crosslinking metal cation complexes break down whereby the cation is reacted and/or lost prior to penetrating into the formation to the in-depth location required.
U.S. Pat. No. 4,799,548 issued on Jan. 24, 1989 discloses two crosslinkers with differing temperature responsive crosslinking characteristics which are used to gel water soluble polymers for the purpose of diverting steam in steam stimulation operations carried out in heavy crude oil wells. The high temperature activated crosslinking agents disclosed are comprised of combinations of phenolic compounds and aldehydes and are generally suitable for altering the water permeability of subterranean formations having temperatures of 210.degree. F. or above.
Thus, there is a need for methods and compositions for altering subterranean formation permeability which can be effectively utilized at temperatures in the range of from about 160.degree. F. to 210.degree. F.